Paper is formed from a stock containing less than one percent paper fibers by weight. The fibers contained in the stock are deposited on a forming fabric and a web is formed by draining water from the stock through the forming fabric. In many modem papermaking machines stock is injected between two forming fabrics in a so-called twin wire former. The web, however formed, is dewatered in three sections of the papermaking machine. The sections are referred to as the forming section, the pressing section, and the drying section. The paper web typically leaves the forming section with a fiber content of ten to twenty-five percent fiber by weight. The-paper web leaves the pressing section with a fiber content of between thirty-five and forty-five percent fiber by weight. Finally the paper web is dried to about ninety-five percent fiber dry weight in the drying section.
The direction of progress in the papermaking industry is to improve paper quality while reducing cost. Cost is reduced by increasing the speed at which paper is manufactured and by decreasing the amount of fiber required for a web with selected properties. Costs are also reduced by decreasing the amount of energy used in forming and drying the paper web. Quality is improved by better control over fiber supply, and the processes used in forming the finished paper.
Greater speed complicates the control of the processes by which the paper web is dried. Thus greater machine speeds drive a search for new and better processes.
Water removed in the drying section is the most costly water removed from a paper web. If a paper web is formed from stock containing one percent fiber, then approximately 99 pounds of water must be removed to form one pound of finished paper web. The last pound of water removed from a paper web which is being formed, which represents taking the web from fifty percent dry weight to ninety-five percent dry weight, is typically accomplished by evaporation in the drying section of a papermaking machine. This last pound of water costs as much to remove as the first ninety-eight pounds.
Thus methods of improving the dewatering processes in the forming section and pressing section are to be sought. In the pressing section the use of extended nip presses and high temperature pressing techniques has increased the amount of water which can be removed by a combination of pressure and temperature. In the forming section, where water is typically removed by drainage and vacuum, new methods of increasing water removal are needed.
One approach is to pass air through the web to draw or blow water from between the fibers which form the web. Air can be drawn by a vacuum, but vacuum has two limitations. First, the process takes place in Earth's atmosphere, and thus the maximum vacuum is limited to less than sea level pressure. Second, practical and cost considerations limit the cost-effective levels of vacuum obtainable in practice to considerably less than 14.7 psi.
The use of vacuum to draw water from a paper web is a fairly straightforward process. A box is placed on the side of a forming fabric opposite a paper web and air is drawn from the box. The low pressure pulls the forming fabric against the box forming a seal. The vacuum also controls the amount of pressure or force with which the forming fabric presses against the box and any fabric supports bridging the box.
If pressure is used, sealing the pressurized box to the web can be a problem. If the box is held against the forming fabric with insufficient force, air will leak around the box, causing a loss of air and possibly disrupting the web by blowing along the plane of the web. If too much force is used to hold the box against the forming fabric, excessive wear of the fabric results. The fabrics used to form the paper web are expensive and premature replacement of the forming fabrics results in additional costs caused by the lack of productivity while the machine is down. Unlike vacuum, which supplies its own clamping force, pressure requires a separate system to develop the sealing force.
Part of the answer is disclosed in U.S. Pat. No. 5,225,042 to Eaton et al. which discloses how a seal can be formed by pressing a sealing member against an unsupported portion of a forming fabric. Eaton et al. discloses a system useful for pressures to about ten psi. Further, Eaton et al. shows a gravity drain system opposite the pressure box.
Certain grades of paper, such as tissue paper or creped papers are typically formed by pressing the web onto a large diameter Yankee dryer, and creating a soft absorbent web by scraping the web off the dryer surface with a doctor blade. Alternative approaches hold out the possibility of increasing absorbency while overcoming the limitations of using a single large diameter Yankee dryer. If a web can be dried without pressing, an absorbent web can be formed without creping the web with a doctor blade. New approaches may lead to more cost-effective approaches to manufacturing these important and widely used grades of paper.
Critical to improving the manufacture of tissue paper without creping is an ability to reduce water content in the web as formed without compressing the web. The process of supplying high pressure air and vacuum simultaneously to the web in the forming section has the possibility of reducing web water content by three to five or more percent. This represents a significant reduction in cost compared with removing the same water by techniques which are solely dependent on evaporation for reducing the water content of the web.
What is needed is an apparatus which removes water from a paper web with high pressure air which does not disrupt the web and does not lead to excessive wear of the forming fabrics.